A conventional monochrome liquid crystal display (LCD) capable of displaying video images uses a matrix of pixels. A pixel is defined as the smallest discernible picture element. An image is created on the LCD by varying the gray level of each pixel in the matrix. The gray level of all pixels is updated, typically, every 1/60 of a second, or 16.7 ms, which is referred to as a "frame".
A conventional, sunlight readable, color LCD has each pixel subdivided into at least three primary color subpixels-red, green and blue. These subpixels are too small to be perceived separately. Instead, the human eye integrates the pixel area and perceives only the composite pixel. Sunlight readable color is achieved by using a powerful white backlight and locating red, green or blue filters over each subpixel. Varying the gray level of each subpixel in a pixel determines the mix of red, green and blue in a pixel and consequently, its color and brightness. As in a monochrome display, video images are displayed by updating gray levels every frame, i.e. every 16.7 ms. These conventional displays use nematic liquid crystal, including twisted nematic, supertwist and other variants. Gray level control is achieved for these conventional LCD's by controlling the transmissivity of each liquid crystal pixel cell. Using nematics, transmissivity is monotonically related to the electrical charge deposited on each cell, and incremental control of gray level is easily achieved.
A different type of LCD, known as a field sequential LCD (FSLCD), minimizes the power required to produce sunlight readable color LCD's. For the FSLCD, each 16.7 ms frame is further divided into three equal time intervals, or "fields" of 5.55 ms each. During each field, a high efficiency colored light source is used to backlight the liquid crystal display panel, first with red, then with green, and then with blue light.
A field sequential LCD is more efficient than other conventional LCD's because no color filters are used and each color component of the backlight is allowed to pass through the entire pixel area, and not just a subpixel fraction of each pixel. The human eye cannot respond to these fast changes, and so integrates the light within each 16.7 ms. What is perceived is a pixel having the desired composite color and brightness.
Access to each cell in the matrix is enabled by a vertical column, with a pulse of such amplitude to produce a desired gray level being applied via a horizontal row. This pulse is used to charge the cell. (Note that the terms "row" and "column" are interchangeable throughout this disclosure without change in meaning.)
The charging of cells is performed one column at a time, from left to right on the matrix. The gray levels are set first, followed by the backlighting of the cells that have their gray levels set, using a specific colored lamp, then extinguishing of that specific colored lamp, followed by resetting of gray levels for the next specific colored lamp. This process moves as a wave from left to right across the matrix display. Typically, with a three color Red, Green, Blue, backlighting system, the light sources or lamps of one color are interposed with the others as follows: Red, Green, Blue, Red, Green, Blue, Red, etc., resulting in a fairly complex backlighting system.
At any instant, the width of the back of the liquid crystal display panel which the backlight is allowed to illuminate is restricted, so as not to backlight the cells that are set for the preceding color, and/or cells set for a following color. As a consequence of this limitation on active backlight width, the time which an energized backlight is present for any one column is brief, and peak light output required of the backlight is high.
There is a need for a field sequential liquid crystal display in which all of the cells (pixels) in the display are charged approximately simultaneously, so that the backlight of the display is no longer restricted at any one time to a limited number of columns, thereby allowing backlighting of the entire display for any one color to be accomplished by a single lamp which can be energized at a lower intensity for as long as approximately one third of the frame.